Question

Pick out the incorrect statement among the following. (1) ferric alum is commonly known as Mohr's salt (2) cast iron cannot be welded (3) heating steel to a high temperature followed by quick quenching in water is called annealing (4) alnico is used for making utensils and automobile parts. (a) \(1,2,3\) (b) 1,3 (c) 2,4 (d) \(1,3,4\)

Short answer

Option (d) \(1,3,4\) is incorrect.

Step-by-step solution

Understand the Statements

The exercise requires identifying incorrect statements. It's important to first understand what each statement means: 1. Ferric alum being Mohr's salt. 2. Cast iron cannot be welded. 3. Quick quenching of steel is called annealing. 4. Alnico is used for utensils and automobile parts.

Analyze Statement 1

Statement 1 claims that ferric alum is commonly known as Mohr's salt. However, Mohr's salt is actually ferrous ammonium sulfate, not ferric alum. Therefore, this statement is incorrect.

Analyze Statement 2

Statement 2 suggests that cast iron cannot be welded. This is generally true because cast iron is brittle and can be difficult to weld, making this statement correct.

Analyze Statement 3

Statement 3 claims that heating steel to a high temperature followed by quick quenching in water is called annealing. In reality, this process is known as quenching or hardening, and annealing is the process of slow cooling, which makes this statement incorrect.

Analyze Statement 4

Statement 4 states that alnico is used for making utensils and automobile parts. Alnico is actually used for making strong permanent magnets, not for utensils or automobile parts, so this statement is incorrect.

Conclude Incorrect Statements

Based on the analysis, the incorrect statements are 1, 3, and 4. Hence, the answer option that includes these incorrect statements is option (d) \(1,3,4\).

Key concepts

Understanding Ferric Alum and Mohr's Salt

Ferric alum and Mohr's salt are often confused due to their similar appearances, but they are distinctly different compounds chemically. - **Ferric alum**: This is used widely in water purification and dyeing processes. Chemically, it is a type of double sulfate that contains iron, potassium, and aluminum. It is represented by the formula \( ext{K}_2 ext{SO}_4 imes ext{Al}_2( ext{SO}_4)_3 imes 24 ext{H}_2 ext{O} \) where the iron content is typically in the form of ferric ions.- **Mohr's Salt**: Unlike ferric alum, Mohr's salt is known as ferrous ammonium sulfate. This compound is characterized by a greenish-blue color and is primarily used for titration in analytical chemistry due to its stability.To avoid confusion, it is essential to remember their distinct purposes and chemical compositions. Mohr's salt, being a stable ferrous compound, differs significantly from the ferric alum used in industrial applications.

Exploring Mohr's Salt

Mohr's salt, scientifically named ferrous ammonium sulfate, is a double salt of ferrous sulfate and ammonium sulfate. It has widespread applications in the laboratory.- **Chemical Nature**: Its chemical formula is \( ext{(NH}_4)_2 ext{Fe(SO}_4)_2 imes 6 ext{H}_2 ext{O} \). This crystalline compound forms blue-green crystals and remains stable at room temperature, making it ideal for quantitative chemical analysis.- **Applications**: It’s particularly beneficial in redox titration due to its stable nature and defined oxidation state, which reduces measurement errors during experiments.When working with Mohr’s salt, the key is to rely on its stability and known properties, thus making it a standard in precision chemistry experiments.

The Annealing Process Explained

Annealing is a heat treatment process used to alter the physical and sometimes chemical properties of a material, commonly used in metallurgy. - **Process Description**: The process involves heating a material, usually metal, to a specific temperature and then allowing it to cool slowly. This slow cooling transforms the microstructure, relieving internal stresses and enhancing ductility and toughness. - **Critical Differences**: It is crucial to differentiate annealing from quenching. While annealing involves gradual cooling, quenching (or hardening) involves rapid cooling, resulting in a harder and more brittle material. Understanding annealing is vital for applications requiring enhanced material properties such as flexibility and reduced brittleness, which are essential for manufacturing resilient products.

Understanding Alnico Magnets

Alnico is an acronym referring to an alloy composed of aluminum (Al), nickel (Ni), and cobalt (Co), known for creating strong permanent magnets. - **Composition and Properties**: Alnico also contains iron along with copper or titanium to enhance its magnetic properties. It's known for having excellent temperature stability and resistance to demagnetization. - **Uses**: Contrary to the incorrect statement, alnico is not used for making utensils or automobile parts. Instead, it's used in high-performance settings such as electric motors, sensors, and loudspeakers. Its qualities distinguish alnico as an invaluable material in situations demanding reliable and durable magnetism, where both temperature consistency and magnetic strength are prioritized.

Challenges in Cast Iron Welding

While the statement claims that cast iron cannot be welded, welding cast iron is indeed possible, albeit challenging due to its inherent properties. - **Why It's Difficult**: Cast iron is a carbon-rich, brittle material, making it susceptible to cracking and warping when subject to temperature changes from welding. - **Techniques for Welding**: Successful cast iron welding often involves preheating the metal to reduce temperature gradients during the process. Post-weld heat treatment can also help in preventing cracks. The complexity of cast iron welding emphasizes the need for specialized techniques and equipment, making it a field reserved for skilled welders who can manage the material's unique properties.